Cholesteric liquid crystals are well-known for their ability to change color as a function of temperature. For example, the temperature visualization feature of liquid crystals has been utilized to make various devices, such as thermometers. Cholesteric liquid crystals have also been used in the past for medical diagnostic devices. For example, the device shown in FIG. 1, was used to thermally profile a human feet (Shlens M, Stoltz M R, Benjamin A: “Orthopedic Applications of Liquid Crystal Thermography” published in the Western Journal of Medicine; volume 122, pages 367-370; May 1975). To produce such thermal response, each foot was first painted black before liquid crystal (LC) material was applied thereon as a thin viscous film or coating. However, such prior art approach is messy and time consuming to complete. Furthermore, such approach only allows for a single cholesteric formulation to be coated or applied on the person's feet, and therefore limits the temperature range and/or sensitivity of the response. Because of the inconvenience and the limited temperature range and/or sensitivity that was achievable by this approach, it was never widely adopted for use in medical diagnosis.
Cholesteric liquid crystals have also been coated on fibers to produce fabrics that change color with temperature for unique fashion applications. For example, U.S. Pat. No. 4,642,250 entitled “Fabrics and Garments Formed Thereby Having Thermally-Sensitive Chromatic Properties” discloses a technique of micro-encapsulating liquid crystals in a minute, synthetic casing. These encapsulated crystals are dispersed on yarns of a fabric and bonded thereto, so that the liquid crystals will not wash out or be dislodged from the yarns, when the fabric is washed or otherwise cleaned. However, such coated fibers have high thermal sensitivity in only a limited or narrow temperature range, as only one liquid crystal formulation is used. As a result, the usefulness of the temperature measurements provided by such coated fabrics is limited.
Therefore, there is a need for a thermochromic fabric that incorporates multiple cholesteric liquid crystal (LC) formulations each having high thermal sensitivity over a narrow or specific temperature range, so as to allow the thermochromic fabric to have a high thermal sensitivity over a broad temperature range. In addition, there is a need for a thermochromic fabric that is inexpensive, and convenient to use, such that it can be used as a diagnostic medical tool, which is suitable for direct use at home by patients, thereby allowing for the early detection of a wide variety of medical conditions. Furthermore, there is a need for a thermochromic fabric that eliminates the complications and messiness of prior art techniques, whereby patients are first painted with black ink before a film of cholesteric liquid crystal (LC) material is applied thereon.